Ignition and flame detection means for gas burners

ABSTRACT

A burner ignition and flame detection circuit operating on an AC power supply: in which, conduction of an SCR effects the opening of a fuel valve, the heating of a thermal time switch, and the discharge of a capacitor through the primary winding of an igniter transformer to produce an ignition spark; in which, means including an RC timing circuit effects the firing of the SCR at substantially midpoint of its conducting half cycle irrespective of line voltage, and maintains its conduction throughout the remainder of the half cycle; in which, flame conduction responsive circuit means effects substantially instantaneous cutoff of SCR conduction when burner flame appears; and, in which, a current limited holding circuit holds open the fuel valve when SCR conduction is cut off.

United States Patent [72] Inventor Allan W. Lindberg Kirkwood, Mo.

[21] Appl. No. 855,514

[22] Filed Sept. 5, 1969 [45] Patented Oct. 5, 1971 [73] AssigneeEmerson Electric Co.

St. Louis, Mo.

[54] IGNITION AND FLAME DETECTION MEANS FOR GAS BURNERS 6 Claims, 1Drawing Fig.

[52] 11.5. C1 431/69, 431/78 [51] Int. Cl F23n 5/00 [50] Field ofSearch431/25, 7880, 69, 71

[56] References Cited UNITED STATES PATENTS 3,270,799 9/ 1966 Pinckaers431/25 3,291,183 12/1966 Fairley 431/78 X 3,301,307 l/1967 Nishigaki eta1. 431/78 3,439,991 4/1969 Fathauer 431/80 X 3,445,173 5/1969 Malavasietal..... 431/25 3,489,500 1/1970 Giuffrida et a1 431/26 3,501,6523/1970 Thomson 431/79 X Primary Examiner-Carroll B. Dority, Jr.Attorney-Charles E. Markham ABSTRACT: A burner ignition and flamedetection circuit operating on an AC power supply: in which, conductionof an SCR effects the opening of a fuel valve, the heating of a thermaltime switch, and the discharge of a capacitor through the IGNITION ANDFLAME DETECTION MEANS FOR GAS BURNERS This invention relates toautomatic control systems for fluid fuel burners which includeelectrical ignition means for igniting the burner and a safety timeswitch operative to permit the supply of fuel to the burner for apredetermined short trial period during which the igniter is operated,and thereafter provided combustion of the fuel occurs within the trialperiod, and to cut off the supply of fuel if combustion fails to occurwithin the trial period or, if having occurred, subsequently fails.

The US. Pat. No. 3,393,039, issued July 16, 19681968, to R. W. Eldridge,Jr., et al., discloses a burner control system of this kind; in which,conduction of an SCR effects the opening of the fuel valve, the heatingof a thermal time switch, and the discharge of a capacitor through theprimary winding of an igniter transformer to effect sparking acrosselectrodes in a secondary circuit. In the Eldridge et al. system a neonbulb is employed to trigger the SCR, and a relatively slow-responding,temperature responsive switch is employed to cut off conduction of theSCR upon the appearance of burner flame.

While a neon bulb functions satisfactorily as an SCR triggering means,it has been found that variations in the firing voltage of commerciallyavailable neon bulbs, due to manufacturing tolerances, results invariations in the firing angle of the SCR and, consequently, variationsin the length of time current is passing through the time switch heatereach cycle. Also, the use of any voltage responsive triggering means forthe SCR would result in considerable variation in the firing angle ofthe SCR as the line voltage varied, even though the firing voltage ofsuch triggering means is closely controlled. Variations in the length oftime current is passing through the thermal time switch appreciablyaffect the length of the trial period, which in the interest of safetyis made as short as practicable. It is of particular importance whenemploying instantly responsive flame detection means to cut ofi SCRconduction upon the appearance of flame that heating of the time switchbe maintained as uniform as possible so that the safer, shorter trialperiod that an instantaneous flame detector makes possible can be used.

Moreover, whenemploying a neon bulb or other voltage responsive SCRtriggering means, it is necessary to select one with a nominal tiringvoltage safely below the lowest peak supply voltage likely to occur soas to insure operation of the system under conditions of variable powersupply voltage. This, then, results in the firing of the SCR at a linevoltage which .is considerably below the-nominal peak which, in turn,results in less charge on the igniter capacitor and less energy of thepulse through the igniter primary and, consequently, a reduction inignition spark intensity below that which would occur if the SCR werealways fired near peak line voltage.

An object of this invention is to provide a generally new and improvedelectrical control system for fluid fuel burners, in which: the firingof an SCR supplies electrical pulses for the operation of a sparkigniter and conduction thereof effects the opening of an electricallyoperated fuel valve and the heating of an electrically heated thermaltime switch; and in which the firing of the SCR is caused to occur atsubstantially midway of it conducting half cycle irrespective of supplyvoltage.

A further object is to provide an electrical control system for fluidfuel burners, in which: an SCR, when conducting, heats a thermal timeswitch to effect the cutoff of fuel to a burner at the termination of apredetermined trial period in which to efiect combustion of the fuel;and in which novel means responsive to electrical conduction throughburner flame operates to cut off conduction of the SCR, thereby torender the time switch inoperative upon the appearance of burner flame.

Further objects and advantages will appear when reading the followingdescription in connection with the accompanying drawing.

The single FIGURE of the drawing is a schematic illustration of a burnercontrol system constructed in accordance with the present invention.

Referring to the drawing, a gas burner is indicated at 10, having a gassupply line 12 leading thereto. interposed in gasline I2 is a biasedclosed, electromagnetically opened valve 14 including a solenoid winding16 which when sufficiently energized efiects opening of valve 14. Fuelissuing from the ports of burner 10 is ignited by a spark igniter whichincludes a voltage step-up transformer having a primary winding 18, asecondary winding 20, and a pair of spaced electrodes 22 connectedacross secondary winding 20.

The solenoid winding 16 is connected across terminals 24 and 26 of an ACelectrical power supply through a circuit which may be traced asfollows: from terminal 24 through a lead 28, a lead 32, a diode 34, alead 36, an adjustable resistor 38, a lead 40, solenoid winding 16, alead 42, the electrical resistance safety switch heater 44, a lead 46, acurrent limiting resistor 48, the leads 50 and 52, the safety switch 54,the leads 56, leads 58 and 59, thermostat 30, and lead 60 to powersource terminal 26.

The just-described circuit is a valve hold open circuit through whichcurrent flow is unidirectional due to diode 34, and through whichcurrent flow is limited by the resistor 48 to a value which energizessolenoid winding 16 sufficiently to hold open valve 14 once it has beenmoved open, but is less than a value required to energize winding 16sufficiently to effect opening of valve 14 from its biased closedposition. Also, the unidirectional current flow through'thejust-described circuit partially heats the safety switch resistanceheater 44 and the normally closed adjacent birnetal safety switch blade54, but this current flow is insufficient to effect the warping open ofswitch blade 54 irrespective of time.

The primary winding 18 of the igniter transformer is connected inparallel with current limiting resistor 48 through an SCR 62 by theleads 63, 64, and 66. When SCR 62 is conducting, sufficient additionalunidirectional current flows through solenoid winding 16 to effect theopening of valve 14 from its biased closed position. Also, theadditional unidirectional current flow through safety switch resistanceheater 44, which occurs when SCR 62 is conducting, effects the warpingopen of birnetal safety switch blade 54 in a predetermined relativelyshort period of time. The safety switch blade 54 when sufiicientlyheated by resistance heater 44 to warp open is locked in an openposition by the pivoted latch 55, thereby requiring manual resetting.The safety switch 54 is arranged to deenergize the entire system whenopened.

In the absence of burner flame the SCR 62 is caused to fire midway ofeach conducting half cycle and is continuously biased to continueconducting throughout the remainder of each conducting half cycle bymeans which will be hereinafter described. Upon the occurrence of burnerflame, conduction of SCR 62 is cut oti substantially instantly by meanswhich will also be hereinafter described.

Connected in parallel with SCR 62 and with the igniter transformerwinding 18 by leads 68 and 70 is a capacitor 72, which charges duringthat portion of each conducting half cycle prior to the firing of SCR 62and discharges through transformer primary winding 18 upon the firing ofSCR-62, thereby providing a strong energy pulse which passes throughprimary winding 18 and effects an ignition spark across electrodes 22 inthe circuit of transformer secondary winding 20. The capacitor 72charges through the series connected safety switch resistance heater 44,solenoid valve winding 16, adjustable resistor 38, and diode 34, anddischargesthrough a relatively low-impedance loop comprising thetransformer primary winding 18 and SCR 62.

The firing of SCR 62 at substantially peak supply voltage, irrespectiveof variations in supply voltage, is accomplished by employing atransistor 74' which, when fired. fires the SCR, and in providing an RCtiming circuit comprising a capacitor 76, an adjustable resistor 78, anda fixed resistor 80. to produce a firing signal for transistor 74 atsubstantially midway of the half cycle during which SCR 62 may be madeconductive. The capacitor 76 and resistors 78 and 80 areseries connectedacross the power supply by leads 82, 84, 86, 88,

and 90. The transistor 74 is an NPN type having its emitter connected tothe control electrode of SCR 62 by a lead 92, its collector connected toa point between voltage dividing resistors 96 and 98 by a lead 94, andits base connected to a point between capacitor 76 and resistors 78 and80 forming the RC timing circuit. The base of transistor 74 is connectedto the RC timing circuit through a pair of resistors 100 and 102 byleads 104, 106, and 108. The voltage dividing resistors 96 and 98 areconnected across the power source in series with diode 34 and leads 110,l 12, 1 l4, and 116.

The collector of transistor 74 is, therefore, forward biased during theconducting half cycle of SCR 62, and its base becomes forward biasedwhen the charge on capacitor 76 crosses the zero voltage line fromnegative to positive. Inasmuch as the voltage across capacitor 76 lagsthe voltage across resistors 78 and 80 and the line voltage byapproximately 90, the charge on capacitor 76 will become positive atsubstantially midway of the conducting half cycle of SCR 62 and atsubstantially peak line voltage. The values of resistors 80 and 78 andcapacitor 76 are such that a firing signal is applied to the base oftransistor 74 within a few degrees after the charge on capacitor 76becomes positive. Because the positive signal required to firetransistor 74 is quite small, the firing angle of transistor 74 andconsequently that of SCR 62, will remain substantially constant underthe usual supply voltage variations encountered.

The cutoff of SCR conduction upon the appearance of burner flame isaccomplished by shunting the signal voltage applied to the base oftransistor 74 through a P-channel, junction-type, field effecttransistor 1 16 connected in parallel with capacitor 76 by leads 118 and120. The normally conductive FET 116 is biased off, in the absence ofburner flame, by connection of its gate to power source terminal 24through resistors 122 and 124, and leads 126, 128, 130 and 132, andthrough voltage dividing means comprising resistors 136, 138, and 140,which are connected across the power source by leads 142, 144, 146, 92,148, and 150. The FET gate is also connected to the terminal 26 side ofthe power supply through resistor 122, a small capacitor 152, and leads154 and 156. The small capacitor 152 charges during the conducting halfcycle of diode 34 through resistor 124, of high resistance, andmaintains a positive bias on the FET gate through the nonconductive halfcycle of diode 34 in the absence of flame.

The gate of FET 116 is further connected to the right-hand sparkelectrode 22 in the ignition transformer secondary circuit through aresistor 158 and leads 160 and 162. The burner and the terminal 24 sideof the power supply are suitably grounded at 164 and 166, respectively,whereby a small electrical current will be caused to flow during thenonconductive half cycle of SCR 62 and diode 34 from a point of junction169 through lead 160, resistor 158, lead 162 to spark electrode 22,thence through burner flame, when it exists, to burner 10 and to ground,thence through ground to supply terminal 24. Due to flame rectificationa much greater ion flow occurs from electrode 22 to burner 10 than willflow oppositely. The flame-conducted current flowing through thedescribed path sufficiently reduces the positive charge on the FET'gateto permit it to conduct. Conduction through FET 116 shunts the firingsignal applied to transistor '74, thereby cutting off conduction thereofand, consequently, cutting off conduction of SCR 62. lnasmuch asconduction through a burner flame characteristically varies considerablyas the flame flutters, the capacitor 152, besides storing a charge tomaintain a positive bias on FET 116 during the nonconducting half cycle,also functions to filter current flow through the burner flame.

A neon bulb 170 connected between the spark electrode 22 and theterminal 26 side of the power supply by leads 172 and 174 functions toprotect the FET 116 and the circuit wiring from high voltage which mayoccur if the high-tension electrode 22 becomes grounded. A capacitor 176connected across the power source by leads 178 and 180 protects theentire system against transient voltages incidental to switching,

as at the thermostat 30. A diode 182 connected across the solenoid valvewinding 16 in opposite polarity by leads 184 and 186 permits thecontinued flow of current through winding 16 occasioned by the coilinductance and thereby reduces the tendency of the solenoid to chatterat the lower supply voltages.

OPERATION The closing of thennostat 30 in response to a drop in thetemperature of the space being heated by burner 10 effects the supply offuel to burner 10 and the operation of the spark igniter. There being noburner flame FET 116 is biased off, thereby permitting the applicationof a firing signal to the base of transistor 74 as the voltage oncapacitor 76 crosses the zero line from negative to positive midway ofthe conducting half cycle of SCR 62. The firing of transistor 74, inturn, effects the firing of SCR 62. The sum of the unidirectionalcurrents now passing through resistor 48 and parallel connected SCR 62is sufficient to pull open the valve 14 and effect the warping open ofsafety switch blade 54 in short, predetermined period of time.

Since zero voltage across capacitor 76 and maximum supply voltagesubstantially coincide midway of the conducting half cycle of diode 34,and since the transistor 74 is fired within a few degrees after the zerovoltage crossing of capacitor 76, the firing of SCR 62 will occur atsubstantially maximum line voltage. Also, inasmuch as the phaserelationship of the supply voltage and the voltage across capacitor 76remains substantially constant under conditions of supply voltagevariation and inasmuch as the signal voltage required to fire transistor74 is quite small, the firing angle of SCR 62 will remain substantiallyconstant and will occur at substantially maximum line voltageirrespective of line voltage variations, within prescribed limits, ofcourse.

The minimizing of variations in the firing angle of SCR 62 permitscloser calibration of the safety switch heater 44 to provide a shortertrial period and reduces variations in current flow through solenoidvalve winding 16, which encroach upon the required differential betweenpull-open and hold-open current values. Moreover, the firing of SCR 62uniformly midway of the conducting half cycle, and at peak voltage,permits fully charging capacitor 72 and the application of the maximumenergy pulse to the igniter transformer.

Upon the firing of SCR 62 the voltage on the anode side thereofmomentarily falls through zero to a negative value, but the impedancebetween the SCR 62 and terminal 24 is rela tively low and permits arapid recovery. Also, the firingsignal applied through transistor 74persists through this transient condition and throughout the remainderof the conducting half cycle, so that the current flow through SCR 62continues steady.

When burner flame appears a small current will be caused to flow fromjunction 169 through resistor 158 to the righthand electrode 22, and,thence, through the burner flame to ground during the nonconductive halfcycle of diode 34. The circuit controlling the gate of FET 116 being ahigh-impedance network, this small current flow discharges capacitor 152to such negative value during the nonconducting half cycles of diode 34that it does not charge positively sufficiently during the conductinghalf cycles of diode 34 through high value resistor 124 to hold FET 116in a biased off condition.

FET 116, therefore, becomes conductive and shunts the firing signalotherwise applied to transistor 74. Consequently, conduction through SCR62 is cut ofi substantially instantly upon h ppe ran f urner flame. Thefuel valve 14 will now be held open by the current flow through resistor48. The amount of current flowing through resistance heater 44, underthese conditions, will not effect the opening of safety switch blade 54,so the burner will now continue to operate until thermostat 30 opens.

In event the fuel at burner 10 fails to ignite within the predeterminedtrial period following the closing of thennostat 30, the bimetal switchblade 54 will be warped open and latched open, disconnecting the entiresystem from the power source and permitting the biased closed fuel valveto close.

It will be understood that the values of resistors 138 and 136,connecting the gate of SCR 62 to the anode side thereof, aresufliciently high in relation to the value of resistor 140, connectingthe gate to the anode side, to preclude the application of a firingsignal to the SCR 62 when transistor 74 is not conducting. Also, theresistor 124 and capacitor 152 have values in the order of megohms and0.03 mfd., respectively, whereby the very small current conductedthrough the burner flame is sufficient to reduce the charge on capacitor152 below that required to bias off the FET l 16.

It was found that with certain types of burners a greater and moreconsistent flow of current through the burner flame resulted when theburner itself formed one of the spaced electrodes spanning the flame.Consequently, the burner 10 and the supply line (ahead of diode 34) wereconveniently grounded at 164 and 166 to provide a path parallel toblocking diode 34 for flame conducted current to flow during thenonconductive half cycle. Also, with this arrangement, the capacitor 152is required to hold the off bias on the FET gate during thenonconductive half cycle when no flame exists.

It will be understood, however, that with other types of burners,adequate flame conduction may be achieved by employing two electrodes,other than spark electrodes, spaced in the burner flame and inconnecting them between junction point 169 and supply lead 50 in serieswith resistors 124 and 136 across the power supply, whereby a small flowof current through the burner flame during the conductive half cyclewill preclude the application of a biasing ofi' voltage to the FET gatethrough the high-value resistor 124. In this alternate arrangement thecapacitor 152 is eliminated and the grounding of the burner 10 andsupply line at points 164 and 166 is omitted. Also, the thermostat ispreferably moved to the terminal 24 side of the line in this alternatearrangement.

I claim:

1. In a flame detection circuit for fluid fuel burner an AC powersource, a burner, means to supply fuel to said burner, an SCR operativewhen conducting to effect the cutoff of fuel to the burner in apredetermined short period of time in the absence of burner flame, atransistor operative when conducting to fire said SCR and maintainconduction thereof, means including high impedance means connecting thebase of said transistor to one side of said power source and operativeto apply a forward biasing voltage thereto and effect the conduction ofsaid transistor during a portion of the conductive half cycle of saidSCR, a normally conductive FET connected between said transistor baseand the opposite side of said power source and operative when conductingto shunt the forward biasing voltage applied to said transistor base andprevent the firing of said transistor, means including high-impedancemeans connecting the gate of said FET with said one side of said powersource to effect a cutoff bias of said FET during the conductive halfcycle of said SCR, a pair of spaced electrodes forming an airgap andarranged to be bridged by burner flame when it exists, and circuitconnections including said airgap connecting said FET gate to said otherside of said power source and operative to shunt the cutoff bias appliedto said FET gate and permit conduction thereof when flame exists.

2. A flame detection circuit as set forth in claiml in which a capacitoris connected between the gate of said PET and said other side of saidpower source, and a diode having the same polarity as said SCR is seriesconnected with said high-imand said one side of said power source andeffect the sufficient discharge of said capacitor during eachnonconductive half cycle of said SCR when flame exists, thereby topreclude the application of a cutoff bias to said FET gate through saidhighimpedance means during conductive half cycles.

3. In an electrically operated burner control system, an AC powersource, a burner, a bimetal safety switch having an electricalresistance heater, a normally closed solenoid valve having a winding, acurrent-limiting resistor, a solid-state switch having a controlelectrode, and ignition means, circuit connections connecting saidsafety switch, said safety switch heater, said solid-state switch andsaid solenoid winding in series across said power source, circuitconnections connecting said current-limiting resistor in parallel withsaid solid-state switch, means operative in the absence of burner flameto apply a firing signal to said control electrode of said solid-stateswitch each conductive half cycle thereof at substantially peak powersource voltage, said means comprising a resistor and a capacitor seriesconnected across said power source and circuit connections connectingsaid control electrode to a point between said resistor and capacitor,and switching means operative in response to the appearance of flame atsaid burner to render said last-mentioned mentioned means inoperative toapply a firing signal to said control electrode.

4. The burner control system claimed in claim 3 in which said ignitionmeans comprises an igniter transformer primary winding connected inseries with and arranged adjacent to said solid-state switch, acapacitor connected in parallel with said solid-state switch and saidprimary winding, a transformer secondary winding and a pair of spacedspark electrodes connected in series therewith.

5. The burner control system claimed in claim 3 in which said meansoperative to apply a firing signal to said control electrode of saidsolid-state switch further includes amplifying means having an inputlead connected to said point between said resistor and capacitor andoperative in response to an initial small signal voltage appearingacross said capacitor to apply a strong firing signal to said controlelectrode and maintain conduction thereof to substantially the end ofthe half cycle, and in which said means operative in response to theappearance of burner flame to preclude the application of a firingsignal to said control electrode comprises flame responsive switchingmeans operative to preclude the application of any input signal to saidamplifying means when burner flame exists.

6. The burner control system claimed in claim 5 in which said flameresponsive switching means operative when flame is present to precludethe application of any input signal to said amplifying means comprises anormally conductive solid-state switch having a control electrode,circuit connections including high-resistance means connecting saidelectrode to one side of said power source and operative to effect acutoff bias thereto, a pair of spaced electrodes forming an airgaparranged to be bridged by burner flame, and circuit connectionsincluding said airgap connecting said electrode to the other side ofsaid power source whereby conduction through burner flame bridging saidairgap precludes the application of a cutoff bias to said controlelectrode when burner flame exists.

1. In a flame detection circuit for fluid fuel burner an AC powersource, a burner, means to supply fuel to said burner, an SCR operativewhen conducting to effect the cutoff of fuel to the burner in apredetermined short period of time in the absence of burner flame, atransistor operative when conducting to fire said SCR and maintainconduction thereof, means including high impedance means connecting thebase of said transistor to one side of said power source and operativeto apply a forward biasing voltage thereto and effect the conduction ofsaid transistor during a portion of the conductive half cycle of saidSCR, a normally conductive FET connected between said transistor baseand the opposite side of said power source and operative when conductingto shunt the forward biasing voltage applied to said transistor base andprevent the firing of said transistor, means including high-impedancemeans connecting the gate of said FET with said one side of said powersource to effect a cutoff bias of said FET during the conductive halfcycle of said SCR, a pair of spaced electrodes forming an airgap andarranged to be bridged by burner flame when it exists, and circuitconnections including said airgap connecting said FET gate to said otherside of said power source and operative to shunt the cutoff bias appliedto said FET gate and permit conduction thereof when flame exists.
 2. Aflame detection circuit as set forth in claim 1 in which a capacitor isconnected between the gate of said FET and said other side of said powersource, and a diode having the same polarity as said SCR is seriesconnected with said high-impedance means between said FET gate and saidone side of said power source, whereby said capacitor is charged throughsaid high-impedance means during the conductive half cycle of said SCRand holds the charge during the nonconductive half cycle of said SCR inthe absence of flame, and in which said spaced electrodes forming anairgap arranged to be bridged by burner flame are connected between saidFET gate and said one side of said power source and effect thesufficient discharge of said capacitor during each nonconductive halfcycle of said SCR when flame exists, thereby to preclude the applicationof a cutoff bias to said FET gate through said high-impedance meansduring conductive half cycles.
 3. In an electrically operated burnercontrol system, an AC power source, a burner, a bimetal safety switchhaving an electrical resistance heater, a normally closed solenoid valvehaving a winding, a current-limiting resistor, a solid-state switchhaving a control electrode, and ignition means, circuit connectionsconnecting said safety switch, said safety switch heater, saidsolid-state switch and said solenoid winding in series across said powersource, circuit connections connecting said current-limiting resistor inparallel with said solid-state switch, means operative in the absence ofburner flame to apply a firing signal to said control electrode of saidsolid-state switch each conductive half cycle thereof at substantiallypeak power source voltage, said means comprising a resistor and acapacitor series connected across said power source and circuitconnections connecting said control electrode to a point between saidresistor and capacitor, and switching means operative in response to theappearance of flame at said burner to render said last-mentionedmentionEd means inoperative to apply a firing signal to said controlelectrode.
 4. The burner control system claimed in claim 3 in which saidignition means comprises an igniter transformer primary windingconnected in series with and arranged adjacent to said solid-stateswitch, a capacitor connected in parallel with said solid-state switchand said primary winding, a transformer secondary winding and a pair ofspaced spark electrodes connected in series therewith.
 5. The burnercontrol system claimed in claim 3 in which said means operative to applya firing signal to said control electrode of said solid-state switchfurther includes amplifying means having an input lead connected to saidpoint between said resistor and capacitor and operative in response toan initial small signal voltage appearing across said capacitor to applya strong firing signal to said control electrode and maintain conductionthereof to substantially the end of the half cycle, and in which saidmeans operative in response to the appearance of burner flame topreclude the application of a firing signal to said control electrodecomprises flame responsive switching means operative to preclude theapplication of any input signal to said amplifying means when burnerflame exists.
 6. The burner control system claimed in claim 5 in whichsaid flame responsive switching means operative when flame is present topreclude the application of any input signal to said amplifying meanscomprises a normally conductive solid-state switch having a controlelectrode, circuit connections including high-resistance meansconnecting said electrode to one side of said power source and operativeto effect a cutoff bias thereto, a pair of spaced electrodes forming anairgap arranged to be bridged by burner flame, and circuit connectionsincluding said airgap connecting said electrode to the other side ofsaid power source whereby conduction through burner flame bridging saidairgap precludes the application of a cutoff bias to said controlelectrode when burner flame exists.